1. Field of the Invention
The present invention relates generally to a rotational indexing base curve deposition array for molding contact lenses, and more particularly pertains to a toric indexing base curve deposition array for assembling toric base curves and front curves with a prescribed and programmable angular orientation therebetween in a one step operation. The present invention allows a single type of base curve mold to be selectively rotated to a plurality of different angular positions relative to a front curve mold to produce different toric contact lenses having the toric curve axis positioned at a plurality of different angular positions. Although the specific embodiment discussed herein relates to toric contact lenses, the present invention has broader applicability and can be utilized for any lenses wherein the two optical surfaces require different rotational orientations, such as for some aspheric contact lenses, or wherein an inversion mark is desired on a particular angular location on the lens.
2. Discussion of the Prior Art
It is known that for astigmatic subjects, the astigmatic eye forms an image which contains three main regions:
1. The spherical power focuses as a line;
2. The cylindrical power also focuses as a line, perpendicular to the spherical image line;
3. In between the two, a circular image is formed, known as the "circle of least confusion".
Toric contact lenses are normally prescribed for astigmatic patients with either corneal or lenticular astigmatism, and have a cylindrical optical surface/power which is used to correct for astigmatism in a wearer. Statistically, astigmatism usually occurs in people primarily around either the horizontal axis or the vertical axis, but also at varying axial locations with respect thereto. In the prior art a separate type of toric contact lens is required for each different toric optical power and also for each different orientation of the toric cylindrical axis of the contact lens, which are required to accommodate different patients with differing amounts of astigmatism along different axes.
Accordingly, an inventory of toric contact lenses, or plastic molding parts for molding the toric contact lenses, includes a large number of different combinations of toric axis locations and toric optical powers.
In conventional prior art toric lens designs, a single toric surface comprising a major and minor axis is placed in the optical portion of either the front or the base curve surface, usually the base curve surface. In addition, the axes of the toric lens are usually stabilized in relation to the patient's corneal axes through the use of either a prism ballasted/slab-off feature or a double slab-off feature placed on the front surface of the lens.
Currently, toric lenses are manufactured in the prior art with the following design features:
a. a toric curve on the front or base surface of the lens; PA1 b. prism ballast and slab-off features on the front surface of the lens; PA1 c. the non-toric surface is spherical.
These prior art lens designs correct astigmatism adequately only if the axis of the cylindrical power is accurately aligned with respect to the axis of the astigmatic cornea. A misalignment of the axes (greater than 10.degree.) results in a substantial loss of visual acuity.
Conventional toric lens designs require a large number of stock keeping units (SKUs) in inventory (the total number of the different prescriptions which are maintained in stock and can be prescribed) in order to fit a broad astigmatic patient base. For example, current Frequent Replacement Toric lens products are available in 800 stock keeping units per base curve in inventory (40 spherical powers.times.2 cylindrical powers.times.10 different cylindrical axis placements). Such a large number of stock keeping units per base curve in inventory is uneconomical to produce and maintain, particularly for a disposable modality product. The required large number of stock keeping units in inventory arises primarily from the need to provide 10 or more different cylindrical axis placements. Furthermore, any significant misalignment of the cylindrical axis with respect to the axes of the eye normally results in a significant loss of visual acuity, and accordingly accurate alignment of the cylindrical axis is a primary requirement of such lenses.